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Mr. A. Claudet on Binoadar Vision. 



[April 11, 



the same glance the perception of the respective distances. Therefore, to 

 judge of the distances of certain objects in the direction of the line of vision, 

 we are not absolutely obliged to alter constantly the angle of convergence. 

 This is proved by our perception of the two distances of the surfaces of the 

 card while it is revolving ; for it would be impossible that we should alter 

 the angle of convergence to adapt it alternately to the two surfaces while 

 they are turning so rapidly. 



The same angle of convergence kept on one or the other surface is no 

 impediment to our seeing both in a sufficiently distinct manner. 



The whole phenqmenon may be better understood by the illustration 

 given in fig. 6. 



Fig. 6. 



When we converge the optic axes on B, this point, being represented 

 on the centre of both retinae at B' B", is single, but A being nearer is re- 

 presented on the left of the centre of the left retina at A', and on the right 

 of the centre of the right retina at A"; therefore it appears double. 



For the same reason, converging on A, this point is single, but 

 B is double, with this difference — that one image is on the right of 

 the left retina, and the other on the left of the right retina; so that 

 the double images of nearer objects situated at A are represented out- 

 side the centres of the two retinae, and those of further objects situated at 

 B are represented inside the centres of the retinae, and each of these two 

 different sensations brings to our mind the perception of the distance 

 which has produced it. During the revolution of the card we may 

 adapt the convergence either to one or to the other surface and keep it so ; 

 but in every case the letters on that surface will appear single and a little 

 better defined ; and this, with the sensation of double images of the letters 

 on the other surface, will be an indication of their respective distances. 



As I am not aware that the illusion I have described in this paper has 

 ever been noticed before, it has appeared to me that its publication would 

 excite the interest of all those who look for any new fact capable of illus- 

 trating the principles of binocular vision, and showing the wonderful pro- 



